A seamless tube/pipe can be produced by the Mannesmann tube-making process. This tube-making process consists of the following steps:
(1) A round billet heated to a predetermined temperature is subjected to a piercing-rolling process by using a piercing mill (piercer) to form a hollow blank (hollow shell).
(2) The hollow blank is subjected to an elongation-rolling process by using an elongation-rolling mill (for example, a mandrel mill).
(3) The hollow shell thus elongated is sized so as to have a predetermined outside diameter and wall thickness by using a sizing mill (for example, a stretch reducer).
The piercing mill used in the piercing-rolling step among these steps includes a plurality of angled rolls, a plug, and a pusher as principal components. The pusher is provided on the entrance side along the pass line. The plug is disposed on the exit side along the pass line. The angled rolls are provided around the plug so as to face to each other in a state of having a predetermined crossing angle with respect to the pass line and inclination angle to the horizontal.
In the piercing-rolling using the piercing mill, a billet heated in a heating furnace is supplied along the pass line, and the rear end thereof is pushed by the pusher. Thereby, the billet is conveyed along the pass line toward the angled rolls and the plug, and the front end thereof is engaged into the angled rolls. Thereafter, the billet engaged into the angled rolls advances along the pass line while being rotated by action of the angled rolls. At this time, by the rotary forging effect, Mannesmann fracture is generated successively in the central portion of the billet until the billet reaches the plug nose, and thereby the core part of billet becomes torn off in a brittle manner. Successively, the billet is subjected to a wall-thickness rolling work process by the plug contacting with the central portion thereof and the angled rolls contacting with the outer circumference thereof, and thereby the hollow blank is formed.
The piercing-rolling is performed on an as-continuously-cast billet, which is cast to have a circular cross section, or a billet which is rolled to have a circular cross section by subjecting a cast slab to hot working such as blooming. In the core portion of such a billet, center segregation and porosity more or less occur, and especially for an alloy such as a stainless steel, they occur remarkably. Therefore, on account of the center segregation and porosity in the billet, a leaf-like, fin-like, or lap-like flaw (hereinafter, referred to as an “inner surface flaw”) is generated on the inner surface of the pierced hollow blank by the rotary forging effect at the time of piercing-rolling and by the additional shearing deformation caused by the wall-thickness rolling work.
The prior art for preventing the occurrence of inner surface flaw at the time of piercing-rolling is disclosed in the Patent Literatures described below.
Patent Literature 1 discloses a method for producing a seamless tube/pipe, in which the rotational speed of the angled rolls is restricted to less than 4.5 m per second in circumferential speed, and a back pressure is applied to the billet by the pusher until the billet is securely engaged into the angled rolls. Further, in the technique disclosed in this Literature, piercing-rolling is performed under the condition that a plug nose draft “d/D” is 97% or more, the plug nose draft being defined as the ratio of the angled rolls opening “d” at the plug nose position to the billet diameter “D”.
Patent Literature 2 discloses a pusher which applies a back pressure to the billet until the billet is securely engaged into the angled rolls, and the pushing speed and back pressure of which are variable. Also, this Literature describes that it is necessary to increase the back pressure generated by the pusher with the increase in a draft in terms of the ratio of “roll opening at plug nose position to billet diameter”, which is defined in the same way as a plug nose draft described in Patent Literature 1, and gives 98%, 98.5%, 98.9%, and 99.1% as the draft.
Patent Literature 3 discloses a method for producing a seamless tube/pipe, in which during the time period between when the billet is engaged into the angled rolls and comes into contact with the plug and when the piercing-rolling becomes in a steady state, the billet is pushed by the pusher so that the traveling speed of billet is not less than the traveling speed in a steady state. Further, in the technique disclosed in this Literature, piercing-rolling is performed under the condition that the ratio “Dg/d” of the gorge portion diameter “Dg” of the angled roll to the billet diameter “d” is not less than 4.5, and the gorge draft ratio “Rg/d”, which is defined by the ratio of the gorge portion roll opening “Rg” to the billet diameter “d”, satisfies the specified range in accordance with the piercing-rolling ratio “L1/L0”, which is defined by the ratio of the pierced hollow blank length “L1” to the billet length “L0”.
Patent Literature 4 discloses a method for producing a seamless tube/pipe, in which piercing-rolling is performed under the conditions that the plug nose rolling reduction ratio (TDF) defined by Formula (a) is not more than 0.04, and/or the square root “(TDF×N)0.5” of the product of the plug nose rolling reduction ratio (TDF) and the billet rotational speed “N” is not more than 0.4, and the gorge rolling reduction ratio, which is defined in the same way as the gorge draft ratio described in Patent Literature 3, satisfies the specified range in accordance with the piercing-rolling ratio that is the same as that of Patent Literature 3:TDF=1−(d1/Bd)  (a)where, in Formula (a),
Bd: billet diameter [mm], and
d1: angled roll opening [mm] at plug nose position.
Further, in the technique disclosed in this Literature, piercing-rolling is performed by using a plug having such a geometry that the ratio “L2/d2”, given that “L2” is the length of the rolling portion of the plug, and “d2” is the outside diameter of the reeling start point of the plug, satisfies the specified range in accordance with the square root “(TDF×N)0.5” of the product of the plug nose rolling reduction ratio (TDF) and the billet rotational speed “N”. At this time, the billet is pushed by the pusher at least until the piercing-rolling reaches a steady state from an unsteady state.